Usually thin film solar cells are deposited on substrates exhibiting a TCO layer, acting as the electrode of a photovoltaic (PV) cell. Such TCOs are textured to result in a diffuse scattering of light into the thin film absorber layer. The absorber layer is responsible for the conversion of light into electric energy and the diffuse light scattering into the absorber layer, brought about by the textured TCO, enhances the optical path length leading to a higher absorption and thus an improved light trapping that consequently results in an increased efficiency of the PV cell.
An improved light trapping allows for the reduction of the layer thickness of the active absorber layer, keeping the photocurrent in the cells still high. In turn, a reduced absorber layer thickness effects directly the deposition time and, moreover, reduces the light-induced degradation of amorphous Si solar cells. Therefore, as the light scattering ability correlates with the surface texture and roughness of the substrate, an increased surface texture of the TCO is highly desirable.
“Textured TCO” in the context of this application is understood as TCO exhibiting a surface to ambience or interface to an adjacent material resulting in a scattering of light and a haze of the medium-to weakly absorbed light in the photoactive layer of at least 10%. The texturing may result as (a) as-grown, natural effect of a chosen deposition process, (b) from specially designed process environment and -parameters resulting in an increased texturing compared to (a), (c) a post-treatment of a deposition according to (a) and/or (b).
However, conventional deposition processes often lead to an imperfect coverage of the textured TCO substrate by the absorber layer. Particularly, it is known that conformal coverage of layers deposited by the plasma enhanced chemical vapor deposition (PECVD) technique are low and that the layers of the absorber layer deposited by PECVD cover first the highest peaks of the textured TCO and are not well deposited in the depth of the valleys of the highly textured TCO layers, resulting in local photoactive layer thickness variations. Consequently, the subsequent TCO contact deposition results in an electrical contact between the uncovered TCO zones, as depicted in FIG. 1. This direct electrical contact creates undesired current drains within the device leading to device performance losses.
Furthermore, other sources of shunts, such as particles or imperfect laser scribing may likewise result in such direct electrical contacts and thus a poor module performance.